A thermal printer is an apparatus which heats the back of an ink ribbon superimposed on a recording paper by means of a thermal head in order to thermally transfer the ink of the ink ribbon onto the recording paper, to thereby print an image on the paper. The ink ribbon is a thermal transfer sheet including a layer of thermally meltable color ink, and the recording paper is an image-receiving sheet such as a sheet of paper or a plastic sheet.
The thermal head is a composed of a plurality of heat-generating resistors formed on a substrate in a row. The thermal printer includes a plurality of ink ribbons corresponding to a plurality of colors. The inks of the ink ribbons of the plurality of colors are transferred in a superimposed state onto the recording paper at the same position, to thereby perform color printing. For example, the ink ribbons are disposed to be rotatable, and an ink ribbon to undergo thermal transfer is moved to the position of the thermal head. Further, a recording-paper feed apparatus feeds the recording paper to the position of the thermal head, which is a printing position, whereby printing is effected in a predetermined image-printing area of the recording paper.
There have been known various methods for forming a hairline pattern, which is a pattern composed of a large number of thin lines (hairlines) extending along a specific direction. A method for consistently obtaining a clear hairline pattern has been known (see, for example, Patent Document 1). In this method, hairlines and hairline gaps are arranged on a drawing line of interest; a determination is made as to whether the hairline gaps arranged on the drawing line of interest are adjacent to the hairline gaps of a determined drawing line on which hairlines and hairline gaps have already been arranged; the drawing line of interest is treated as a determined drawing line only when none of the hairline gaps of the drawing line of interest is located adjacent to the hairline gaps of the determined drawing line; and the generation and arrangement of hairlines and hairline gaps for the drawing line of interest are performed once more when any of the hairline gaps of the drawing line of interest is located adjacent to the hairline gaps of the determined drawing line.
There have been known various methods for forming a circular hairline. However, when a fine circular hairline is formed, a moiré, which is a periodic striped pattern, is generated.
There has been known a method for preventing periodic image disturbances, such as moiré and pitch variation, from being emphasized when a plurality of colors are superimposed (see, for example, Patent Document 2). The method is used in an image forming apparatus which forms a color image by repeating an image forming process including a step of applying an oscillating voltage to an electrifier so as to charge an image bearing body. According to the method, the frequency of the oscillating voltage applied to the electrifier is shifted each time the image forming process is repeated, whereby the periodic image disturbances are prevented from being emphasized.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2002-221781
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. H6-202445